The present invention comprises an improvement in the energy economizing AC power control system described in Parker and Hedges, U.S. Pat. No. 4,190,793 issued Feb. 26, 1980, the disclosure of which is incorporated herein by reference.
Conventional induction motors maintain the full sine wave of voltage across the stator winding regardless of the load on the motor. In those cases where the load varies within wide limits, e.g., when the motor is used for hoisting operations, most of the time the motor is not expending its full rated load. In such cases, the iron losses in the stator are substantially the same when the motor is operating below full rated load as is the case when the motor is operating at full rated load; and, due to the low power factor in such cases, the stator current is high and the copper losses are also substantial. Moreover, the efficiency with which a motor converts electrical energy to mechanical energy is also a function of the magnitude of source voltage applied to the stator winding. It is well-known that maximum inherent motor efficiency is achieved only when a particular relationship is realized between applied stator voltage and the rotor load of a moment. Therefore, when the AC power source voltage rises above or sags below about ten percent of its nominally rated value, a common event when said source is distributed over power lines throughout a community from a central power generating utility, motor operating efficiency will decrease proportionately in such instances even when the rotor load is constant.
When a conventional induction motor is operating below its full rated load or under conditions of higher than nominal stator voltage, a fraction of the sine wave voltage would satisfy the actual load requirement imposed on the motor. Such cutting in part of the sine wave voltage would result in considerably less iron and copper losses and less heating of the stator. The resultant low operating temperature further reduces the copper losses in the motor due to lowered ohmic resistance. These factors combine to effect a significant reduction in the energy which is consumed by the induction motor, with a consequent conservation in available energy sources and a reduction in motor operating costs.
The aforementioned Parker et al U.S. Pat. No. 4,190,793, as well as the present invention, is based upon a recognition of the foregoing factors, and each invention provides a simple yet reliable mechanism operative to cause the electrical energy supplied to the stator and the stator flux density of a standard-unmodified-AC induction motor to become a function of its load demand at any given moment. The present invention accomplishes this by permitting a greater or smaller portion of the sine wave of voltage from a power source to enter the stator as a function of operating efficiency-related characteristics of stator inrush current during each alternation. In other words, the sine wave of the voltage supplied to the motor's stator is modified to suit existing load and AC power source conditions. This results in the reduction of iron and copper losses.
The system described in the aforementioned Parker et al patent employs the speed incidentally achieved by an induction motor when operating with maximum efficiency at zero mechanical load as a particular reference by which motor operating efficiency and the presence and magnitude of a subsequently applied load could be determined. More particularly, in the prior Parker et al system a small induction generator was used to develop a frequency modulated signal which was representative of motor speed and mechanical load at any moment. Specifically, said generator produced an AC signal that was electro-mechanically frequency shift modulated at a 120 Hz rate in proportion to applied loads above zero load, thereby to function as a sensitive load detection means. This prior arrangement has two disadvantages, however, i.e., the need (1) to provide and (2) to mechanically couple an induction generator into the system in such a manner that it is capable of responding to motor rotor shaft movements. In the embodiment disclosed in the said Parker et al patent, a separate-signal load detection means is employed, comprising a small induction generator that is mechanically attached to the rotor of the motor being controlled. Employing optical or other mechanical means for motor shaft speed-change measurements would change the specific nature of, but not overcome, those twin disadvantages.
The present invention is characterized by the provision of an improved induction motor efficiency monitoring and load detection means in a system of the general type described in the said Parker et al patent, which overcomes both of the disadvantages noted above. As will be described, the present invention does not require separate generation of a load-modulated and/or speed-related signal and therefore eliminates the need to provide or couple a separate signal generating means of any kind to the motor rotor.